The present invention relates to a hydrogen feed system, in particular but not exclusively to a hydrogen feed system for a fuel cell arrangement in which the hydrogen can be taken at higher pressure from a supply tank standing under pressure or under pressure from a reforming device and can be fed after relaxation to a lower pressure into the fuel cell arrangement, with a return flow loop being provided in the hydrogen circuit, so that a part of the non-consumed hydrogen emerging from the fuel cell arrangement can be fed back into the latter.
Fuel cells are known in diverse forms. The present invention is, however, only concerned with fuel cells which operate with hydrogen as a fuel. Such fuel cells are known in the form of so-called PEM fuel cells (Proton Exchange Membrane fuel cells). A fuel cell arrangement based on PEM fuel cells can admittedly consist of a single fuel cell, but normally consists of a stack of fuel cells arranged above one another or alongside one another, which together form a so-called stack. Each fuel cell has a proton permeable membrane with electrodes on both sides, and indeed a cathode and an anode, which both have a catalytic coating. Hydrogen is supplied to the stack at the anode side at a certain overpressure, i.e. pressure above atmospheric pressure. At the cathode side air is likewise supplied to the stack with a suitable overpressure. In the operation of the fuel cell protons, which are delivered by the hydrogen, diffuse through the membrane and react at the cathode side of the membrane with the air that is supplied. In this way, water vapor is formed on the one hand which is led away as an exhaust gas at the cathode side and, on the other hand, current is produced, which can, for example, be used to drive a vehicle in which the fuel cell arrangement is incorporated.
At the anode side of the fuel cell arrangement excess hydrogen, i.e. not yet consumed hydrogen, leaves the stack and is either burned in order to produce heat or is at least partly returned to the stack via a return loop, that is to say recirculated. A procedure of this kind has certain advantages. It is on the one hand more economical and, on the other hand, the return of the hydrogen ensures that adequate moisture is always present, so that the membranes remain moist. This is an important precondition for the disturbance-free operation of a fuel cell.
It is thus known to return at least a part of the non-consumed hydrogen emerging from the fuel cells to the fuel cell arrangement again. In order to achieve this, a pressure increase must, however, take place, because the pressure at the inlet side of the fuel cell arrangement is higher than at the outlet side. This pressure increase, however, proves to be problematic. Since hydrogen molecules are small, the pumping of hydrogen is difficult and the danger of leakages is very great. Motor-operated pumps, in which the motor is arranged outside of the hydrogen circuit, are problematic in practice because it is extremely difficult to adequately seal the rotating drive shaft of the pump. Hydrogen leakages are, however, extremely dangerous, particularly when a vehicle is stationary, because they can lead to an ignitable gas mixture.
In order to avoid the use of such pumps, it has already been proposed, in the international application with the publication no. WO 99/05741, to use so-called eductors. These consist of a nozzle with a convergent section and a divergent section and hydrogen is injected at a higher pressure into the narrow throat between the convergent section and the divergent section, whereby a suction action takes place with an increase in pressure, so that hydrogen is sucked in at a lower pressure at the convergent side and emerges with a pressure increase at the divergent side. An eductor has the advantage that it can be relatively easily sealed, since there are no moving parts. It is, however, problematic that the eductor first functions correctly with a certain throughput, so that a second eductor is necessary in order to maintain the hydrogen circulation at low flow rates. In the second eductor water is injected into the narrow throat between the convergent and the divergent section for which a water supply and a water pump are required and it is clear that the duration of the operating time is restricted because the available quantity of water is restricted. Through the different components that are required a system of this kind also proves relatively complex and leakages must also be feared here.
The object of the present invention is to provide an apparatus which makes it possible to obtain the pressure increase required for a hydrogen recirculation at a relatively favorable cost without having to fear leakages.
In order to satisfy this object, provision is made in accordance with the invention, in a hydrogen feed system of the initially named kind, for a pump to be provided which brings about the recirculation of the hydrogen and which can be driven by the pressure energy of the hydrogen taken from the tank or coming from a reforming unit.
Viewed differently, the solution of the invention consists in the provision of a pump which is connected to the fuel cell arrangement in order to carry out the hydrogen recirculation; in the hydrogen coming from the supply tank or from a reforming unit being supplied to the pump to drive the pump, and in the hydrogen which is relaxed by the driving of the pump being supplied together with the recirculated hydrogen to the fuel cell arrangement.
Since the recirculation of the hydrogen takes place by a pump operated by the hydrogen itself, in particular in the form of a displacement pump, this pump can be fully accommodated within the hydrogen circuit, so that the pump is fully integrated into the line system and no shaft passes through the wall of the line system, so that leakages at shaft lead-throughs cannot occur. All line connections must admittedly be sealed against hydrogen now as previously, this problem can, however, be solved substantially more simply, because no movable parts are present at the line connections in operation. The pump is, so to say, hermetically sealed off relative to the environment.
Since the drive energy is obtained from the hydrogen pressure which is in any event present, through the relaxation of the hydrogen pressure, no additional energy need be supplied, so that the power yield from the fuel cells is not reduced by the energy required for the driving of the pump.
Preferred embodiments of the invention can be found in the subordinate claims and also in the further description and in the accompanying drawings.